Answer:

We know less than we would like about the long-term effects of narcotics. We do know that they are safe in terms of not damaging the various organs, such as the kidneys, liver or stomach; we limit the amount of Vicodin you take not because of the narcotic, but because of the acetaminophen. However, there are some long-term effects of opioids, including Avinza (morphine), which do occur.

Treatment depends in part upon the cause of the neuropathy. In general, physicians prescribe drugs
that alter the processing of pain information in the spinal cord. Examples would include antidepressants, anti-epilepsy drugs, certain alpha-2 agonists, and opioids.

Opioids in men can cause a decrease in testosterone, so that if you are taking opioids and have a decreased sex drive, testosterone levels should be checked. Long-term use of opioids can be associated with tolerance to the drug or even hyperalgesia, in which the opioids themselves cause the pain to become worse. There is also some evidence to suggest that opioids may decrease immune function. The clinical importance of this evidence has not yet been fully evaluated.

Finally, there is always the question of the long-term effects on the ability to think clearly and to function. The evidence is contradictory on this point; most pain physicians feel that opioids should be used in a way that allows clear thinking and functioning.
In short, doctors believe that opioids are safe for long-term use, but there are many questions which still must be more fully investigated.